This invention relates to novel methods for the production of waterglass compositions, to the compositions produced by such methods and to fire resistant glass laminates comprising such compositions as an intumescent layer between two opposed panes of glass.
Glass laminates incorporating an intumescent inorganic layer sandwiched between two opposed panes of glass are known and are sold under the Trade Marks PYROSTOP and PYRODUR by the Pilkington group of companies. When such laminates are exposed to a fire the inorganic layer intumesces and expands to form a foam. The foam provides a thermally insulating layer which protects the pane of glass remote from the fire so that the structural integrity of the glass unit is maintained and thereby presents a barrier to the propagation of the fire. The insulating properties of the foam layer reduce the amount of heat transmitted through the layer and thereby reduce the risk of combustion of materials on the non-fire side of the glass unit. Glass laminates incorporating such inorganic layers have been successfully used as fire resistant glass structures. Such laminates may comprise more than two glass panes sandwiching more than one inorganic layer. Depending upon the stringency of the relevant regulation laminates comprising as many as eight inorganic layers,have been used. The multi-layered laminates are relatively thick and correspondingly expensive.
The intumescent inorganic layer is normally formed mainly from a sodium silicate waterglass or a mixture thereof with a potassium silicate waterglass. In addition the layer may comprise a minor quantity of a polyhydric organic compound such as a glycol, glycerine and its derivatives or a sugar. The inorganic layer is normally formed by preparing an aqueous solution of the waterglass, applying the solution to one of the panes of glass and drying the excess water so as to form the inorganic layer. Typically the inorganic layer will comprise approximately 20% by weight of water (including water of hydration) and varying amounts of Na2SiO2, Na6Si2O7 and Na2Si3O7. The waterglass solutions and the inorganic layer may contain varying ratios of Na2O to SiO2 and may also comprise potassium containing species by introduction of K2O as an alternative to the Na2O.
The drying step must be carried out under carefully controlled conditions if an optically clear layer having useful intumescent properties is to be obtained. Typically the drying is carried out at high humidity and over a long period say from 12 to 24 hours. This lengthy process adds to the cost of the manufacturing process and reduces the productivity of the production facility.
U.S. Pat. No. 4,190,698 discloses glass laminates in which the interlayer is a silicate waterglass and proposes a variety of adjuvants to the waterglass including glycerine, ethylene glycol, sorbitol, glucose, starch, sodium phosphate, sodium aluminate aluminium phosphate, borax, boric acid and colloidal silica. These adjuvants are stated to improve the fire resistant performance of the interlayer.
The adjuvant which is most commonly added to a silicate waterglass to form an interlayer having advantageous properties is glycerol. The presence of glycerol reduces the cracking of the interlayer during the drying process and thereby facilitates the formation of a clear dried interlayer.
The fire resistance of these glazings comprising these interlayers depends on part upon the integrity of the glazing being maintained for as long as possible even whilst the glass panes are cracking under the stress. In some instances the glass cracks in a catastrophic fashion and the glazing may fail the particular test as a result. We have observed that these failures may be associated with the formation of a relatively coarse foam when the interlayer is heated.